A PEQ? It does one of three things. Some are made to hold a battery. So if you can't put a battery anywhere else in your gun, you can put one in a PEQ. Some are made to somewhat mimic the function of a real PEQ unit, so they have a light, a laser, or both. A real PEQ is a complex and sophisticated visible and IR aiming device. Airsoft versions usually have a laser and/or a flash light. The third thing they can do is look pretty and nothing else. I have a non-functioning PEQ on my M4 because it was dirt cheap and makes my gun look more like what's issued to US soldiers in Afghanistan.

I tend to buy stuff like this used. I got one for like, $10 and the other for free from a friend. If you search the classified sections they come up a lot. If you buy new and you just want one entirely for looks, buy a battery box that doesn't include the battery. It's just a plastic shell, so it's the cheapest way to get one.